The field of medical ultrasonic imaging traditionally uses a linear array of elements (flat or in a convex geometry) to scan a two dimensional image of organs of the body. Arrays deposed of large numbers of very tiny elements are needed in a two dimensional arrangement, so that the acoustic beam can arbitrarily be steered in all directions to achieve three dimensional images of organs, such as the heart, at high frame rates. The acquired data can then be displayed in two dimensional slices by off line processing, taken from arbitrary, user determined, slices through the organ.
The goal is to have high density arrays, xe2x80x9c1.5 Dxe2x80x9d (elevation focusing and aberration correction processing) and 2-D arrays (3-D imaging), with fine pitch elements in both the horizontal and vertical directions, with fine pitch (0.25 to 2.0 wavelengths). Typical 2-D arrays have upwards of 50xc3x9750 elements. There are several obstacles to achieving this type of resolution. There is a lack of available working space and constraints on the ease of bringing signal leads out of the sides of the array. Because of these constraints, a more desirable way of bringing signal leads out of the array involves individual connection directly behind each element, where backing layers are present. Thus, electrically contacting the elements has to be done through the backing mass itself.
One prior method of constructing a two dimensional ultrasonic array requires drilling holes in an epoxy block to line up lead wires behind the array elements, and using silver epoxy to make electrical contact. This process typically is very time consuming, and therefore is not feasible for volume production of a commercial product.
Another technique entails the use of copper/beryllium etched wires, strung out and tensioned by special apparatus. The spacing of the leads to align with the array elements was achieved by a combination of spacer plates and mechanical tensioning devices. The leads were buried in the backing but terminated at the top surface of the backing block. This then required bulky attachment to a cable via various clamping means. This prototype was a bulky machine that was difficult for a clinician to hold between the patient and the scanning table to acquire the desired line of sight view of the organ being examined.
Aside from constructing the array of elements is the consideration of the materials used in the backing of the array. The backing typically has conflicting demands and requirements made on it. The backing must provide adequate mechanical support during the manufacture of the array, and this typically requires a stiffer, more rigid material. The backing must also absorb the acoustic energy coupled into the backing, so it must be highly attenuating, and this typically means softer durometer characteristics to increase visco-elastic damping. Backings are traditionally composed of powder loaded polymers, such as epoxy.
One object of the current invention is to provide a composition for a multi-element ultrasonic imaging array, and an economical means for fabricating the multi-element ultrasonic imaging array in volume for commercial purposes. Another object is to provide a staged backing for the transducer array which will impart both the necessary mechanical rigidity as well as the necessary acoustic absorbency required.
In one embodiment, the present invention is directed to a multilayer backing for an ultrasonic transducer array comprising at least two layers in contact with each other. The first layer is formed from a first composition comprising an epoxy resin base and a powder. The second layer is formed from a second composition comprising an epoxy resin base, a powder, and at least one of a phenolic shell micro-balloon and a plasticizer, preferably both a phenolic shell micro-balloon and a plasticizer. The first layer is more rigid than the second layer so that the first layer provides mechanical support for the array, while the second layer provides maximal sound absorbance. The multilayer backing can optionally contain a third layer in contact with the second layer. The third layer is formed from a third composition comprising an epoxy resin base and a powder, with the third layer being more rigid than the second layer. The third layer also provides mechanical support.
In another embodiment, the invention is directed to a method for making a backing for an ultrasonic transducer array. The method comprises setting a circuit into a mold. A first backing material composition, as described above, is poured into the mold to form a first layer. A second backing material composition, as described above, is poured into the mold to form a second layer in contact with the first layer. The first layer is more rigid than the second layer. The method can further include pouring a third backing material composition, as also described above, into the mold to form a third layer in contact with the second layer. The first backing material composition is cured, either before or after the second backing material is poured.
A more complete understanding of the backing material and method for its manufacture will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description.